1. Field of the Invention
The present invention relates to a display apparatus which comprises organic electro-luminescence devises individually having an organic light emitting layer, and a method for fabricating such display apparatus.
2. Description of the Related Art
An organic electro-luminescence (abbreviated as EL hereinafter) device based on electro-luminescence of organic materials has an organic layer comprising an organic hole transport layer and an organic light emitting layer laminated between a lower electrode and an upper electrode, and is attracting a good deal of attention as a light emitting device capable of high-luminance light emission by low-voltage direct current drive.
Since such an organic EL device can respond as fast as within 1 μsec or below, an organic EL display apparatus using such devices can be driven by simple-matrix duty operation. A problem will, however, arise in that there is a growing need for instantaneously applying a large current to the organic EL devices to ensure sufficient luminous intensity in the future situation in which the EL display apparatus will be operated at a higher duty responding to an increasing trend in the number of pixels, which is likely to damage the devices.
On the other hand in active-matrix operation, operation current can constantly be applied to the organic EL devices during a period of one frame according to signal voltage, since the signal voltage can be retained by a retention capacitor provided to the individual pixels together with a thin film transistor (abbreviated as TFT hereinafter). Thus, unlikely to a case with the single matrix operation, there is no need to instantaneously apply a large current so that the damages to the organic EL devices can be reduced.
In the active-matrix display apparatus using such organic EL devices (that is, the organic EL display), each pixel on a substrate is individually provided with a TFT, the TFTs are covered with an interlayer insulating film, and further thereon organic EL devices are formed. Each organic EL device comprises a lower electrode patterned for the individual pixels so as to be connected to the TFT, an organic layer formed so as to cover the lower electrode, and an upper electrode provided so as to cover the organic layer.
In such an active-matrix display apparatus, the upper electrode is formed as a so-called blanket film covering all pixels, and used as an upper common electrode for such all pixels. In the display apparatus capable of color display, the organic layers are separately patterned for each color on the lower electrode.
Such display apparatus having organic EL devices on the insulating film covering the TFTs on the substrate is, however, disadvantageous in that such TFTs are causative of narrowing the aperture of the organic EL devices when the display apparatus is designed as of transmission type in which light emitted from the organic layer is observed from the substrate side.
Thus for the active-matrix display apparatus, it is supposed to be advantageous to adopt a so-called upper light withdrawing structure (hereinafter referred to as top emission type) in which the light is withdrawn from an opposite side of the substrate so as to ensure a sufficient aperture of the organic EL devices.
Constituting the active-matrix display apparatus as the top emission type requires the lower electrode to be made of a light reflective material and the upper common electrode to be made of a transparent material. Indium tin oxide (ITO) and indium zinc oxide (IXO), known as materials for transparent conductive film, are however higher in resistivity than metals, so that the upper common electrode is likely to cause voltage drop due to internal voltage gradient occurs therein. This may cause significant degradation of the display properties since the voltage applied to the individual organic EL devices on the display plane becomes non-uniform, and since the luminous intensity in the center portion of the display plane decreases.
While the transparent conductive film such as ITO or IXO can be formed by evaporation or sputtering, the former method is difficult to yield a film of good quality, and the obtained film tends to have high resistivity and low light transmissivity. The sputtering is thus adopted in the fabrication process of the display apparatus to form the transparent conductive film. The sputtering is, however, higher in energy of particles to be deposited as compared with vapor deposition, so that the underlying surface on which the deposition proceeds is more likely to be damaged. Since the basic structure of the organic EL device resembles to that of a light emitting diode made of inorganic semiconductor materials, such damages exerted on the underlying organic layer will be causative of leak current, which may further result in the production of non-emissive pixels called “dead pixels”.
To avoid the production of such dead pixels, it is proposed to form the upper common electrode with a metal film thin enough so as to allow sufficient light transmissivity. Such metal film may, however, inevitably has a high sheet resistance as a result of the thinning and will also produce voltage gradient in the upper common electrode similarly to the case with the transparent conductive film, which will cause voltage drop and thus will significantly lower the display properties.
The thinning of the upper common electrode raises another problem in that the electrode cannot fully prevent the atmospheric moisture or oxygen from intruding into the organic layer, which will accelerate deterioration of the organic layer.